In recent years, an image forming apparatus such as a copying machine and a printer, in which an electrophotographic process is employed, has widely been used. Such an image forming apparatus includes, for example, a photoreceptor drum, a charging unit for charging the photoreceptor drum at a predetermined electrical potential, a laser scanning unit for emitting laser light in accordance with image data accepted from the outside and generating an electrostatic latent image on the photoreceptor drum, a developing unit for supplying toner to the electrostatic latent image formed on the surface of the photoreceptor drum so as to visualize the image, and a transferring unit for transferring the toner image formed on the surface of the photoreceptor drum onto a paper. The image forming apparatus allows the user to form a desired image on a recording medium by the electrophotographic system.
In such an image forming apparatus, due to non-uniformity in a charging characteristic depending on a position of the photoreceptor drum, an amount of laser light emitted from the laser scanning unit, a toner mass per unit area in the developing unit, and a difference in distance between the developing unit and the photoreceptor drum, for example, unevenness occurs in density for the image formed on, for example, a recording medium, which may further cause non-uniformity in density depending on a forming position. If such non-uniformity in the density is not resolved, the density of an image may be different for each forming position even if the image is formed on a recording medium based on image data which is to be formed with uniform density.
In order to resolve non-uniformity in density depending on a forming position, Japanese Patent No. 3405055 discloses a method of correcting image data by printing a test patch, performing colorimetry on the printed test patch and obtaining an amount of correction for each position to correct image data.